Cannot access outlook.com accounts in Outlook 2016

Microsoft seems to be in process of updating our old custom domain addresses on their back end. I have about 15 of these addresses I've used for years. I'm now starting to have failures accessing these accounts in Outlook 2016 clients and on iPad/iPhones however they are still accessible via the web and using the Outlook App on idevices. Any ideas? I'm sure this is somehow tied to the MX record for my domain as some addresses that have not been migrated by Microsoft are still working, some that have been migrated work, others don't. Any help would be appreciated